There is a requirement to refine the microstructure of a titanium aluminide alloy, in particular cast titanium aluminide alloy, which does not involve hot working of the titanium aluminide alloy.
Our published European patent application EP1378582A1 discloses a method of heat-treating a titanium aluminide alloy having a single alpha phase field and being capable of producing a massively transformed gamma microstructure. In that method of heat-treating the titanium aluminide alloy is heated to a temperature above the alpha transus temperature, is maintained above the alpha transus temperature in the single alpha phase field for a predetermined time period, is cooled from the single alpha phase field to ambient temperature to produce a massively transformed gamma microstructure, is heated to a temperature below the alpha transus temperature in the alpha and gamma phase field, is maintained at the temperature below the alpha transus temperature for a predetermined time period to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced and is then cooled to ambient temperature.
A problem with this heat-treatment is that the cooling, quenching, of the titanium aluminide from above the alpha transus to ambient temperature induces quenching stresses in the titanium aluminide. The quenching stresses may result in cracking of castings. A further problem is that the heat-treatment is only suitable for relatively thin castings.
Our published European patent application EP1507017A1 discloses a method of heat-treating a titanium aluminide alloy having a single alpha phase field and being capable of producing a massively transformed gamma microstructure. In that method of heat-treating the titanium aluminide alloy is heated to a temperature above the alpha transus temperature, is maintained above the alpha transus temperature in the single alpha phase field for a predetermined time period, is cooled from the single alpha phase field to a temperature in the range 900° C. to 1200° C. to produce a massively transformed gamma microstructure, is heated to a temperature below the alpha transus temperature in the alpha and gamma phase field, is maintained at the temperature below the alpha transus temperature for a predetermined time period to precipitate alpha plates in the massively transformed gamma microstructure such that a refined microstructure is produced and is then cooled to ambient temperature.
In this heat-treatment the cooling, quenching, of the titanium aluminide from above the alpha transus to a temperature in the range 900° C. to 1200° C. reduces quenching stresses in the titanium aluminide and hence reduces cracking of castings. The heat-treatment is suitable for thin castings and for thicker castings.
Cracking during cooling, quenching, from a temperature above the alpha transus temperature, is related to both cooling rate and the dimensions of the titanium aluminide castings. Generally, cracking is promoted by relatively high cooling rates and by relatively large dimension castings.